Downslope Windstorms Downslope Windstorms and Rotors Jonathan Vigh AT707 21,23,25 February 2005
Aspects of flow over barriers Aspects of flow over barriers 2D mountain waves Flow over/around isolated hills Blocking by large amplitude mountains Gap flows, funneling Cold air damming, barrier jets Föhn and Bora winds Downslope windstorms Thermally-driven circulations (slope winds, mountain winds, valley winds), katabatic winds Orographic control of precipitation Quasi-geostrophic flow over a mountain Lee cyclogenesis Gravity wave drag
Naming conventions and Naming conventions and terminology What are the different types of downslope winds and where do they occur? Föhn winds (warming winds) Föhn (Alps), Chinook (Rockies), zonda (eastern Andes), pulche (western Andes), Santa Ana (southern California), sundowner (Santa Barbara, CA), berg (South Africa), koschava and ljuka (Croatia), germich (SW Caspian Sea), afganet and ibe (Central Asia), kachchan (Sri Lanka), Canterbury north-wester (New Zealand) Bora winds (cooling winds) bora (Croatia), mistral (France) Glenn (1961) concluded that there was no satisfactory definition for a Föhn flow Yoshino (1975) said that the definition should simply be based on whether the temperature of the downslope flowing air was warmer or cooler than the air it was displacing Chinooks and boras do not always have severe winds It is best to just call this phenomenon downslope windstorm or severe downslope windstorm
Gap flows between mountains of Europe Föhn and bora winds of the Western U.S.
A few notes on the Chinook A few notes on the Chinook Why are Chinooks warm? The descending air can be warmer for one or more reasons: Compressional heating as air descends the lee side of the mountain (air warms according to the dry adiabatic lapse rate: 9.8 C per km of descent) Latent heat of condensation (Hahn effect): air ascends moist adiabatically (cooling ~5 C per km) on windward side of mountain while precipitation is produced, then descends dry adiabatically on the leeward side Blocking on the windward side of the mountain can cause air to descend from a higher level (higher θ) Displacement of cold air by warmer air Nighttime mixing Other peculiarities can occur if a cold pool is present Waves can propagate on the inversion surface (see Black Hills paper) Mirages can make the mountains look higher Figures from Whiteman (2000)
Notable Boulder Windstorms 7-8 January 1969 130 mph at NCAR, 96 mph downtown 1 death 23 January 1971 147 mph at NCAR 11 January 1972 wind gusts to 97 mph 40 trailers damaged $3 million damage 4 December 1978 148 mph at? 1 death 17 January 1982 137 mph at NCAR 20 gusts above 120 mph in 45 min! At least 15 injuries $20 million damage, 40% of Boulder buildings damaged! 24 January 1982 140 mph at Wondervu 24 September 1986 131 mph at? $100,000 damage 24 January 1992 143 mph (kt?) north Boulder 2-3 February 1999 127 mph at Sugarloaf, 120 mp in Lafayette, 119 mph in Wondervu, 100 mph in Longmont, 98 mph in Boulder $3 million damage 8-10 April 1999 120 mph at Sugarloaf, near 100 mph in south Boulder, 90 mph in Longmont $20 million damage from Pueblo to Fort Collins Photos courtesy of UCAR Damage in South Boulder from the 8 January 1969 windstorm (Ed Zipser) From 1967-1995, winds > 100 mph occurred 35 times Source: http://www.co.boulder.co.us/sheriff/pdf/oem/highwind.pdf and http://www.cdc.noaa.gov/boulder/wind.html
Windstorm Damage Windstorm Damage According to RMIIA, Colorado's top five most costly windstorms are: $20 million in insured damage occurred along the Front Range on April 8-10, 1999. $20 million in insured damage occurred in Boulder County on Jan. 17, 1982. $10 million in insured damage occurred along the Front Range on Jan. 28-29, 1987. $5.2 million in insured damage occurred along the Front Range on Oct. 29, 1996. $3 million in insured damage occurred along the Front Range on Feb. 2-3, 1999.
Observations from 1968 Field Project Observations from 1968 Field Project Figure from Kuettner and Lilly (1968)
Figure from Kuettner and Lilly (1968)
Data from a composite study Data from a composite study Figures from Brinkmann (1974)
Composite soundings at onset of Composite soundings at onset of Boulder windstorms Note that base of isothermal level is 575 in the upstream sounding, but is lower (650 mb) in the downstream sounding. Also, downstream sounding has steeper lapse rate, higher θ at lower levels Figures from Brinkmann (1974)
Composite Denver soundings for Composite Denver soundings for different types of windstorms Downstream sounding composites Inversion level averages ~675 mb for Boulder wind cases ~625 mb for slope wind cases Figures from Brinkmann (1974)
Observations from the 11 January Observations from the 11 January 1972 Windstorm Figure from Lilly (1978)
Figure from Lilly (1978)
Figures from Lilly (1978)
Pressure traces during a windstorm Note in particular the sudden pressure drop at Boulder during the storm Is this causing the high winds? Figure from Lilly and Zipser (1972)
Downslope Windstorm Downslope Windstorm Mechanisms There are 3 proposed mechanisms for downslope windstorms: 1. Develop when the flow over a mountain transitions from subcritical -> supercritical over the mountain, analogous to a hydraulic jump 2. Large-amplitude verticallypropagating mountain waves that undergo partial reflection at interfaces properly tuned waves resonate with increasing amplitude 3. Wave breaking and waveinduced critical layers Vertically propagating waves with large amplitude Trapped lee waves undergoing reflection Figures from Whitefield (2000)
Long s Hydraulic Jump (1953a) Long s Hydraulic Jump (1953a) Homogeneous fluid flowing over ridge-like obstacle. Assume flow is in hydrostatic balance and bounded by free surface. Consider y-independent motions Assume steady-state flow. u u x + g D x + g h x = 0 Where D is the thickness of the fluid and h is the obstacle height. Using the continuity equation 1 ( D + h ) h ( ud ) (1 ) = Most people Interpret Fr 2 = 0 Fr x x 2 as the Froude #. Here it is we get: a ratio of the fluid speed to x where the propagation speed of shallow linear gravity waves 2 2 u Fr = gd So the free surface can either rise or fall depending on the magnitude of Fr 2
Supercritical case Supercritical case Fluid thickens and slows as it crosses top of obstacle, minimum speed at crest Schematic from Durran (1990)
Subcritical case Subcritical case Fluid thins and accelerates as it crosses top of obstacle, reaches maximum speed at crest Schematic from Durran (1990)
Hydraulic jump case Hydraulic jump case Flow transitions from subcritical to supercritical at top of obstacle, potential energy is converted to kinetic energy over the entire barrier Figure from Long (1953a) Schematic from Durran (1990)
Forecasting There appear to be at least three mechanisms which can cause the flow to undergo transition from subcritical to supercritical: 1. Wave breaking - in an atmosphere with constant N and u 0 ; mountain large enough to cause breaking waves (Clark and Peltier 1977) 2. Scorer-parameter layering in an atmosphere with constant u 0 and two layers of N: mountain too small to cause breaking waves (Durran 1986a) 3. Capping by a mean-state critical layer: in an atmosphere with constant N and u 0 below a critical layer, where in the absence of the critical layer, the mountain is too small to cause breaking waves Other mechanisms: wave-induced critical layers?
General development General development characteristics For the case of deep cross-mountain flow and no meanstate critical layer, observations suggest a windstorm will occur when: Wind is directed across mountain (within 30 of perpendicular to ridgeline) and wind at mountaintop level exceeds a terrain dependent value of 7 to 15 m s -1 Upstream temperature profile exhibits an inversion or a layer of strong stability near mountaintop level (Colson 1954; Brinkmann 1974) These conditions favor development of a downslope storm by creating conditions similar to Scorer parameter layering. They also promote the development of larger amplitude mountain waves, increasing the chances for breaking waves. Breaking waves are favored when the upper tropospheric winds are not too strong.
The effect of a mountaintop The effect of a mountaintop inversion Figure from Durran (1990)
Other forecast factors Other forecast factors Ideal terrain for windstorms are long ridges with gentle windward slopes and steep lee slopes (effective terrain shape can be modified by upstream blocking) Low humidity is better (moisture seems to reduce amplitude) Nighttime or early morning more likely (stability changing during this time?) Klemp and Lilly (1975) found that the strongest downslope events occur in Boulder when a one-half wavelength phase shift was present between ground and tropopause (partial reflection mechanism of linear theory) Durran (1986a) ran simulations of the 11 January 1972 event this condition appears necessary, but not sufficient for strongest windstorms Elevated inversions might be required for breaking waves to form Lee et al (1989) found that the presence of cold pools in the lee of the mountain could have a strong determinant in whether downslope winds would make it to the mountain base it also altered the overall structure of the mountain wave in simulations (different lower boundary, change in scale?) In some cases, precipitation effects could play a role? (e.g. the 3 July 1993 Fort Collins windstorm case)
25 October 1997 Blowdown Event 25 October 1997 Blowdown Event West of the Park Range 13,000 acres of old growth trees blown down in Mount Zirkel Wilderness Area/Routt National Forest, trees stacked 30 feet high, hunters trapped for 2 days; strongest winds lasted ~30 min Wind gusts exceeded 100 mph for 7 hrs at Arapahoe Basin Ski Area (el. 12,500 ft; peak gust 114 mph out of the east, windchill to -60 F) Factors: strong synoptically-driven flow from east to west (blizzard of 97), an unusually cold and stable layer on the windward side of the mountains Picture courtesy U.S. Forest Service For more, see Meyers et al (2003)
RAMS Model Simulation Figures from FSL Forum, Feb. 1999 (Wind speed in knots, height in meters)
Historical Fort Collins Windstorms Historical Fort Collins Windstorms Fort Collins Windstorms noted in conjunction with Boulder Windstorms (as reported in the Boulder Daily Camera) Dec. 4-5, 1880 Mar. 18, 1920 May 21, 1925 also including Eaton, Greeley, Windsor, and Platteville Jan. 15, 1943 including Loveland, Ft. Morgan Dec. 20, 1948 average wind of 41 mph, gust to 96 mph Dec. 6, 1963 Fort Collins, Greeley, Sterling Jan 27-28, 1965 gust to 73 mph Feb. 13-14, 1967 Dec. 6, 1967 gust to 66 mph Dec. 21-22, 1969 Larimer County Nov 30-Dec 1, 1970 Mar 31, 1971 Ft Collins and Laporte, 40-50 mph, gusts to 72 mph, one fatality and several injuries in Fort Collins Jan 11-12, 1972 Fort Collins Loveland and just about everywhere Nov 25-26, 1972 Fort Collins Data from Whiteman and Whiteman (1973)
Fort Collins Windstorm Climatology Fort Collins Windstorm Climatology Peak windstorm season is during the winter months Windstorms often occur in streaks ; Jan. 1977 had 7 separate events! Summer events are rare, but not unprecedented (June 1973, July 1993) Fort Collins Windstorms, 1977-1989 20 18 16 14 12 10 8 6 4 2 0 Number of windstorm events Most or all of the data is from old Foothills campus station > 54 kt > 70 kt January February March April May June July August September October November December Data courtesy of John Weaver (see Lee et al 1989; Weaver and Phillips 1990) Month
For comparison, the seasonal cycle of Boulder winds Source: http://www.bcna.org/winds.html
FC Windstorm Climatology, cont d FC Windstorm Climatology, cont d 65 windstorms with max gusts >54 kt (~5 per year) 12 windstorms with max gusts >70 kt (~1 per year) Fort Collins Windstorms 12 10 8 6 4 2 0 > 54 kt > 70 kt Number of windstorm events 1977/78 1978/79 1979/80 1980/81 1981/82 1982/83 1983/84 1984/85 1985/86 1986/87 1987/88 1988/89 Data courtesy of John Weaver (see Lee et al 1989; Weaver and Phillips 1990) Year
Max gusts in kt (y-axis) versus time of day occurs (x) 85 80 75 70 65 60 55 50 0 6 12 18 24 Note that there is no preferential time of year for maximum gusts Max gusts occur during all parts of the day, with a weak peak around midday 85 Max gust in kts versus Month of year 80 75 Average windstorm duration is 8 hrs (min of 1.25 h, max of 20 h) 70 65 60 Data courtesy of John Weaver (see Lee et al 1989; Weaver and Phillips 1990) 55 50 0 1 2 3 4 5 6 7 8 9 10 11 12
Wind trace of windstorm at Fort Collins Note that winds are much steadier than Boulder storms Trace courtesy of Richard Johnson
Notable Fort Collins Windstorms Notable Fort Collins Windstorms Most or all of the data is from the old Foothills campus station, data after 1988 is from Christman Field station Date 17/18 Jun 1973 26 Nov 1977 3 Jan 1982 24 Jan 1982 29/30 Mar 1982 2 Apr 1982 30 Mar 1983 Strength of max gust (kt) 79 71 74 83 74 78 72 15 Feb 1986 73 4/5 May 1986 78 28/29 Jan 1987 73 2 Dec 1987 73 18 Sep 1988 79 22 Dec 1988 77 3 Jul 1993 82 Data courtesy of John Weaver (see Lee et al 1989; Weaver and Phillips 1990)
Rotors Rotor: a region of reversed flow Figures from Doyle and Durran (2004)
A Unique Observation A Unique Observation Figure from Grubiši and Lewis (2004) On another occasion (25 April 1955) a unique observation of the rotor circulation was made when the Sierra Wave Project sailplane broke apart in severe turbulence near the upwind edge of the roll cloud and the pilot, Mr. Larry Edgar, descended through this region by parachute. After being carried rapidly down the direction of the main stream eastward across the Valley below the roll cloud, he encountered a layer of calm air at about 2,500 m (1,300 m above the ground) below which he drifted westward in a wind estimated at 25 knots. He finally landed on the west side of the Owens Valley below the leading edge of the roll cloud. -- Quote from Scorer and Klieforth (1958)
Figure from Grubiši and Lewis (2004) Rotor observations from Sierra Wave Project
Figure from Grubiši and Lewis (2004)
Figure from Grubiši and Lewis (2004)
Rotor Formation Rotor Formation Queney (1955) proposed a simple cat s eye formation mechanism for rotors Transformation of a stationary wave motion into a system of vortices in the vicinity of a level where the basic wind velocity is vanishing Figure from Queney (1955)
Rotor formation, cont d Rotor formation, cont d Doyle and Durran (2002) have a recent paper on rotor dynamics: Kinematic considerations suggest that boundary layer separation is a prerequisite for rotor formation. Numerical simulations suggest that boundary layer separation is greatly facilitated by the adverse pressure gradients associated with trapped mountain lee waves and that boundary layer processes and lee-wave induced pressure gradients interact synergistically to produce low level rotors. Mechanical shear in the planetary boundary layer is the primary source of a sheet of horizontal vorticity that is lifted vertically into the lee wave at the separation point, and partly carried into the rotor. Realistic rotors appear to only develop in the presence of surface friction. Surface heat flux above the lee slope increases the vertical extent of the rotor circulation and the strength of the turbulence but decreases the magnitude of the reversed rotor flow.
Figure from Doyle and Durran (2004)
Figure from Doyle and Durran (2004)
References Blier, W., 1998: The Sundowner winds of Santa Barbara, California. Wea. Forecasting, 13, 702--716. Brinkmann, W. A. R., 1971: What is a foehn? Weatherwise, 26, 230--239. Brinkmann, W. A. R., 1974: Strong downslope winds at Boulder, Colorado. Mon. Wea. Rev., 102, 592--602. Cotton, W. R., J. F. Weaver, and B. A. Beitler, 1995:An unusual summertime downslope wind event in Fort Collins, on 3 July 1993. Wea. Forecasting, 10, 786--797. Doyle, J. D., and D. R. Durran, 2002: The dynamics of mountain-wave-induced rotors. J. Atmos. Sci., 59, 186--201. Doyle, J. D., and D. R. Durran, 2004: Recent developments in the theory of atmospheric rotors. Bull. Amer. Meteor. Soc., 85, 337--342. Durran, D. R., 1986a: Another look at downslope windstorms. Part I: The development of analogs to supercritical flow in an infinitely deep, continuously stratified fluid. J. Atmos. Sci., 43, 2527--2543. Durran, D. R., 1986b: Mountain waves. Mesoscale Meteorology and Forecasting, Ray, P. S., Ed., Amer. Meteor. Soc., 472--492. Durran, D. R., 1990: Mountain waves and downslope windstorms. Atmospheric Processes over Complex Terrain, Meteor. Monogr., No. 45, Amer. Meteor. Soc., 59-- 81. Gafn, D. M., 2002: Unexpected warming induced by foehn winds in the lee of the Smoky Mountains. Wea. Forecasting, 111, 907--915. Glenn, C. L., 1961: The Chinook. Weatherwise, 14, 175--182.
References, Cont d Hamann, R. R., 1943: The remarkable temperature fluctuations in the Black Hills region, January 1943. Mon. Wea. Rev., 71, 29--32. Hertenstein, R. F., and J. Kuettner, 2002: Simulations of rotors using steep lee-side topography. 10th Conf. on Mountain Meteorology, Amer. Meteor. Soc., Park City, UT, 330--333. Hugh D. Cobb, III, 2004: The Gulf of Tehuantepec hurricane force wind event of 30-31 March 2003. Mar. Wea. Log, 48, 7--9. Jones, C. N., J. D. Colton, R. L. McAnelly, and M. P. Meyers, 2002: An examination of a severe downslope windstorm west of the Colorado Park Range. Natl. Weather Dig., 26, 73--82. Julian, L. T., and P. R. Julian, 1969: Boulder's winds. Weatherwise, 22, 108--112, 126. Klemp, J. B., and D. K. Lilly, 1975: The dynamics of wave-induced downslope winds. J. Atmos. Sci., 32, 320--339. Kuettner, J., and R. F. Hertenstein, 2002: Observations of mountain-induced rotors and related hypotheses: A review. 10th Conf. on Mountain Meteorology, Amer. Meteor. Soc., Park City, UT, 326--329. Kuettner, J. P., 1968: Lee waves in the Colorado Rockies. Weatherwise, 21, 180--185, 193, 197. Lee, T. J., R. A. Pielke, R. C. Keesler, and J. Weaver, 1989: Influence of cold pools downstream of mountain barriers on downslope winds and flushing. Mon. Wea. Rev., 117, 2041--2058. Lessard, A. G., 1988: The Santa Ana winds of southern California. Weatherwise, 41, 100- -104.
References, Cont d Lilly, D. K., 1978: A severe downslope windstorm and aircraft turbulence event induced by a mountain wave. J. Atmos. Sci., 35, 59--77. Lilly, D. K., and E. J. Zipser, 1972: The Front Range windstorm of 11 January 1972. A meteorological narrative. Weatherwise, 56--63. Long, R. R., 1953a: Some aspects of the flow of stratified fluids. I. A theoretical investigation. Tellus, 5, 42--58. Long, R. R., 1953b: Steady motion around a symmetrical obstacle moving along the axis of a rotating fluid. J. Meteor., 10, 197--203. Long, R. R., 1954: Some aspects of the flow of stratified fluids. II. Experiments with a two fluid system. Tellus, 6, 97--115. Long, R. R., 1955: Some aspects of the flow of stratified fluids. III. Continuous density gradients. Tellus, 7, 341--357. Manley, G., 1945: The Helm Wind of Crossfell, 1937-1939. Quart. J. Roy. Meteor. Soc., 71, 197--219. Meyers, M. P., J. S. Snook, D. A. Wesley, and G. S. Poulos, 2003: A Rocky Mountain storm. Part II: The forest blowdown over the West Slope of the northern Colorado mountains--observations, analysis, and modeling. Wea. Forecasting, 18, 662--674. Peltier, W. R., and J. F. Scinocca, 1990: The origin of severe downslope windstorm pulsations. J. Atmos. Sci., 47, 2853--2870. Pirazzoli, P. A., and A. Tomasin, 1999: Recent abatement of easterly winds in the northern Adriatic. Int. J. Climatol., 19, 1205--1219. Queney, P., 1955: Rotor phenomena in the lee of mountains. Tellus, 7, 367--371.
References, Cont d Raphael, M. N., 2003: The Santa Ana winds of California. Earth Interactions, 7, 1--13. Riehl, H., 1971: An unusual Chinook case. Weather, 26, 241--246. Rotunno, R., and P. K. Smolarkiewicz, 1995: Vorticity generation in the shallow-water equations as applied to hydraulic jumps. J. Atmos. Sci., 52, 320--330. Scorer, R. S., 1949: Theory of waves in the lee of mountains. Quart. J. Roy. Meteor. Soc., 75, 41--56. Scorer, R. S., and H. Klieforth, 1958: Theory of mountain waves of large amplitude. Quart. J. Roy. Meteor. Soc., 84, 131--142. Smith, R. B., 1979: The influence of mountains on the atmosphere. Advances in Geophysics, Saltzman, B., Ed., Vol. 21, Academic Press, 87--230. Smith, R. B., 1985: On severe downslope winds. J. Atmos. Sci., 42, 2597--2603. Grubiši, V. and J. M. Lewis, 2004: Sierra Wave Project Revisited: 50 Years Later. Bulletin of the American Meteorological Society, 85, 1127 1142. Weaver, J. F., and R. S. Phillips, 1990: An expert system application for forecasting severe downslope winds at Fort Collins, Colorado, U.S.A. 16th Conf. on Severe Local Storms, Amer. Meteor. Soc., Kananaskis Park, AB, Canada, 13--15. Whiteman, C. D., 2000: Terrain-forced flows. Mountain Meteorology: Fundamentals and Applications, Oxford Univ. Press, 141--170. Whiteman, C. D., and J. G. Whiteman, 1973: An historical climatology of damaging downslope windstorms at Boulder, Colorado. NOAA Technical Report ERL 336- APCL 35, NOAA/ERL, Boulder, Colo. 62 pp. Wolyn, P. G., 2003: Mountain-wave induced windstorms west of Westcliffe, Colorado. 10th Conf. on Mountain Meteorology, Amer. Meteor. Soc., Park City, UT, 402--405. Yoshino, M. M., 1971: Die bora in jugoslawien: Eine synoptischeklimatologische betrachtung. Ann. Meteor., 5, 117--121.